This invention relates to an improved reactor and process for producing methanol utilizing catalyst tubes which are catalytically inert with respect to the methanol synthesis gases and have approximately the same thermal expansion properties as the reactor in which the methanol synthesis is carried out. It is known to produce methanol by the reaction of a synthesis gas, which contains oxides of carbon, and hydrogen, in contact with catalysts which contain zinc and chromium and under pressures above 300 kilograms per square centimeter above atmospheric pressure and at temperatures of about 320.degree.-400.degree. C. The reactors used for this purpose are cylindrical pressure vessels, in which the catalyst packing is divided into a plurality of layers. Cold fresh synthesis gas and/or cooled recycled synthesis gas is introduced between the catalyst layers so that the heat of reaction is partly consumed. Owing to the high reaction pressure, considerable energy is required to compress the synthesis gas. If the synthesis gas is available under a pressure of 16 kilograms per square centimeter, about 408 kWh are required per metric ton of methanol product for the compression to the synthesis pressure of 325 kilograms per square centimeter above atmospheric pressure.
In contact with copper-containing catalysts, synthesis gases having the above-mentioned composition can be reacted to form methanol with good yields under lower pressures and at lower temperatures.
It is known to perform the reaction in contact with catalysts which contain copper, zinc, and, if desired, chromium under pressures of 50-120 kilograms per square centimeter and at temperatures of 230.degree.-280.degree. C. In that case too, shaft reactors are used, in which the catalyst layer is divided and cold gas is introduced between the layers. The compression of the synthesis gas from 16 kilograms per square centimeter above atmospheric pressure to the working pressure of 100 kilograms per square centimeter above atmospheric pressure still requires an energy of 302 kWh per metric ton of methanol product.
It is known to effect the synthesis of methanol in contact with copper-containing catalysts and at the lower reaction pressure and temperature conditions in tubular reactors, in which the catalyst is disposed in tubes which are surrounded by boiling water under pressure. This arrangement of the catalyst results in a very favorable temperature control of the highly exothermic reaction, and the entire heat of reaction is utilized in the production of highpressure steam under a pressure of 35-50 kilograms per square centimeter above atmospheric pressure. If this high-pressure steam is used to drive the compressors for compressing the fresh synthesis gas and/or the recycled synthesis gas, only 54 kWh of extraneous energy are required per metric ton of methanol for the compression.
Some by-products are formed in the synthesis of methanol. In addition to water, these are mainly dimethyl ether, methyl formate, iron carbonyl, higher alcohols and also higher hydrocarbons. The crude methanol produced in the synthesis is purified in most cases by a two-stage distillation. In a first column for the first runnings, those impurities which have a lower boiling point than methanol are distilled off as an overhead product, and in a second, pure methanol column the pure methanol is distilled off as an overhead product from the bottoms product of the first column whereas the higher-boiling impurities remain back as bottoms product.
It has been found that the formation of these by-products is due only in part to the catalyst and particularly the formation of hydrocarbons is due to the catalytic activity of the reactor wall. Specifically, it has been found that the formation of just these by-products is promoted in tubular reactors in which the catalyst is disposed in tubes that are surrounded by a coolant, and depends much more on the properties of the selected tube material than in shaft reactors. In a tubular reactor there is a much smaller relation of the volume to the surface of the catalyst layer. When these tubes are made from plain carbon steel or low-alloy carbon steels, which have proved quite satisfactory as a material for shaft reactors, up to more than 2000 ppm hydro-carbons may be found in the methanol product. These hydrocarbons include compounds having a boiling range near the boiling point of the methanol so that their separation from methanol by distillation involves a high expenditure. This disadvantage detracts substantially from the advantage of the water-cooled tubular reactor for the methanol synthesis these advantages reside in the highly effective dissipation of the heat of reaction and in the ability to produce high-pressure steam. It must also be borne in mind that the tubular reactor is more complicated in structure than the simple shaft reactor so that in addition to the catalytic activity, a plurality of other properties of the material must also be taken into account, particularly the coefficient of thermal expansion and the resistance to corrosion by boiling water, which may contain ions.